Gear differentials used in automobiles interconnect an input shaft to two output shafts. The input shaft is connected to a differential housing that is rotatable about a common axis of the two output shafts. Carried within the housing is a planetary gear set that interconnects the two output shafts for opposite directions of relative rotation. Front differentials connect a front (input) drive shaft to two front (output) axle halves, rear differentials connect a rear (input) drive shaft to two rear (output) axle halves, and center differentials connect an input shaft to the front and rear (output) drive shafts.
Ordinarily, the planetary gearing of at least front and rear differentials interconnects the two output shafts at a speed ratio of minus one. At such a speed ratio, a single rotation of one output shaft with respect to the housing produces a single rotation of the other output shaft in an opposite direction with respect to the housing. In this way, the two output shafts can rotate at different speeds, which average to the rotational speed of the housing.
While rotating at such different speeds (i.e., differentiating), input torque is divided between the output shafts in accordance with the efficiency of the planetary gear interconnection. A ratio of the torques between two relatively rotating output shafts is referred to as a "bias ratio". The same ratio of torques (or a little larger) is required to initiate differentiation. Reduced efficiency of the interconnection between output shafts permits uneven torque distributions between the output shafts up to the bias ratio at which differentiation is initiated. Bias ratios of two-to-one or more make better use of uneven traction by preventing lower traction drive wheels from spinning until at least two times more torque is distributed to the higher traction drive wheels.
Automotive differentials operate within a matrix of four different modes of loading. Each of the modes is a combination of a direction of torque transfers between the input and output shafts (i.e., drive or coast loading) and a direction of torque transfers between the two output shafts.
Ordinarily, differentials are thought to exhibit the same bias ratio in all four loading modes. However, some worm gear differentials exhibit bias ratio imbalances in opposite directions of torque transfer between the two output shafts. Such imbalances can be useful in center differentials for favoring torque distributions to either the front drive shaft or the rear drive shaft but are generally undesirable for opposite directions of torque transfer between axle halves of front and rear differentials. Several commonly owned patents deal with such imbalances including U.S. Pat. No. 4,191,071 to Gleasman et al.; U.S. Pat. Nos. 4,805,487 and 4,890,511 to Pedersen; and U.S. Pat. No. 5,232,415 to Brewer et al.
Generally, such bias ratio imbalances have been controlled by regulating either frictional characteristics or loading of gear mounting surfaces that contribute more to the bias ratio in one direction of torque transfer than the other. Increasing friction or loading at such mounting surfaces increases the bias ratio imbalance, whereas decreasing friction or loading decreases the bias ratio imbalance.
Variations in speed ratio between output shafts have also been used to imbalance bias ratios in opposite directions of torque transfer between output shafts. In a frictionless system, torque is distributed between the two output shafts in inverse proportion to the absolute speed ratio. However, friction supports a much wider range of torque distributions between the output shafts, so the overall effect of the change in speed ratio is a bias ratio imbalance between the opposite directions of output shaft torque transfer. Co-owned U.S. Pat. No. 2,859,641, to Gleasman discloses a speed ratio change in a center differential to favor torque distributions to either a front or a rear drive shaft. German Patent 39 06 650 discloses another example with the same objective.
The Brewer et al. patent as well as others including German Patent 40 13 202 have sought to control bias ratios between drive and coast loading directions. Some gear mounting surfaces are loaded in only the drive loading direction and other of the gear mounting surfaces are loaded in only the coast loading direction. Accordingly, varying the frictional characteristics of such mounting surfaces varies the bias ratio in one direction of loading (drive or coast) independently of the bias ratio in the other loading direction.
We have discovered that bias ratios can be independently controlled between any one of the four main loading modes. For example, it may be desirable to favor torque distributions to a front drive shaft in a coast mode and a rear drive shaft in a drive mode, so the maximum torque distributions match vehicle weight distributions.
While it is desirable to achieve such bias ratio control, automotive differentials must be inexpensive, compact, and very rugged to operate effectively at reduced efficiencies. Special care must be taken to distribute loads throughout the differentials to avoid producing excessive wear or strain on any one part.
One gearing arrangement we particularly favor mounts both sun and side gear members of a planetary gear set on parallel axes. The sun gear members, more generally referred to as "side" gears, are coupled to inner ends of the output shafts. The planet gear members of the same set are mounted in pairs. One meshing portion of each planet gear engages one of the side gears, and another meshing portion of each planet gear engages its paired planet gear.
Commonly owned U.S. Pat. No. 5,122,101 to Tseng discloses an example of a parallel-axis gear differential in which the planet gears are formed by two gear sections separated by a stem. One of the gear sections has a first meshing portion engaged with one of the side gears and a second meshing portion engaged with its paired planet gear. The other gear section has a third meshing portion that is also engaged with its paired planet gear. The two meshes between the paired planet gears straddle the two meshes between the planet gears and the side gears to provide for a better balance of loading throughout the differential.
Ordinarily, the two side gears are positioned together between the straddled planet gear meshes. However, the side gears can also be separated to provide room for a fixed spacer or a driving block for connecting a coaxial input shaft to the differential housing. Commonly owned U.S. Pat. No. 5,292,291 to Ostertag discloses one example in which the stem sections of the planet gears are lengthened to straddle the spacing between side gears. However, U.S. Pat. No. 5,492,510, issued to one of the subject inventors, uses the additional planet gear length to support another meshing portion between the planet gears. The new meshing portion can be arranged either as a third meshing portion between planet gears or as a replacement for one of the two straddled planet gear meshing portions.